1. Field of the Invention
The present invention relates generally to solid-state imaging detectors of ionizing radiation and, in particular, to amorphous selenium radiation detectors having a field-shaping multi-well avalanche detector structure.
2. Description of the Related Art
Soon after the Nobel Prize winning invention of the gas-filled multi-wire proportional chamber by Charpak in 1968 and parallel to developments in microelectronics, micro-pattern gaseous detectors were developed for improved position resolution. However, the range of radiation induced photoelectrons is micrometer to millimeter in, with gas solid-state detectors having three-orders-of magnitude shorter photoelectron range due to their much higher density. Thus, solid-state detectors yield images with substantially higher spatial/temporal resolution. Disordered solids, which are easier and less expensive to develop than single crystalline solids, have not been utilized as photon-counting mode detection media because of low carrier mobility and transit-time-limited photo response.
Amorphous selenium (a-Se), which was previously developed for photocopying machines, has been commercially revived as a direct x-ray photoconductor for Flat-Panel Detectors (FPD) due to high x-ray sensitivity and uniform evaporation over a large area as a thick film. However, current direct conversion FPDs are limited by, inter alia, degradation of low-dose imaging performance due to electronic noise, because energy required to generate an electron-hole pair in a-Se is 50 eV at 10 V/micron. Although other photoconductive materials with higher conversion have been investigated, the other photoconductive materials suffer from charge trapping and manufacturing issues. Improved conversion of a-Se is possible by increasing the electric field above 30 V/micron, i.e., 30,000 V on a 1000 micron layer. However, this electric field increase is extremely challenging for reliable detector construction and operation, and is virtually impractical.
Amorphous solids, i.e., non-crystalline solids with disorder, have been ruled out as viable radiation imaging detectors in a photon-counting mode because of low temporal resolution due to low carrier mobilities and transit-time limited pulse response, and low conversion gain of high energy radiation to electric charge. Avalanche multiplication in selenium can be used to increase the electric charge gain. However, significant obstacles prevent practical implementation of a direct conversion a-Se layer with separate absorption and avalanche regions.
A separate localized avalanche multiplication region minimizes gain variation compared to bulk avalanche, i.e., avalanche in an entire volume of a-Se. However, a separate localized avalanche multiplication region has not been realized due to formation of field hot-spots, where F exceeds 150 V/μm, leading to irreversible material breakdown. The concept of unipolar solid-state detectors with a Frisch grid has been proposed, but such structures are not practical for direct conversion avalanche gain because the highest electric field in the well develops at the interface between the semiconductor and the pixel electrode, thereby resulting in high dark current due to large charge injection and potentially irreversible detector damage.
Therefore, provided herein is a novel radiation detector that overcomes disadvantages of conventional detectors.